Project Summary The overall goal of this application is to understand how ion channels in T cells regulate immune responses to pulmonary infection with influenza virus. Influenza is major health risk and affect millions of patients in the US and worldwide. CD4 T cells play a critical role in supporting germinal center B cells to produce neutralizing antibodies against influenza virus and to become memory B cells, which together provide immunity against reinfection. The function of CD4 T cells is regulated by ion channels that mediate the influx of calcium and other ions. The calcium release-activated calcium (CRAC) channel, which is formed by ORAI1 proteins in the plasma membrane, is one of the best characterized channels in T cells. It mediates a specific and essential form of calcium influx, store-operated Ca2+ entry (SOCE), so called because it is triggered by the release of calcium from the endoplasmic reticulum. Ca2+ release activates stromal interaction molecule 1 (STIM1) and STIM2 and results in the opening of ORAI1 CRAC channels. Mutations in ORAI1 or STIM1 genes in human patients that abolish SOCE cause immunodeficiency with recurrent infections due to impaired T cell function and production of pathogen-specific antibodies. This defect is mimicked by STIM1/STIM2 double-deficient mice, whose CD4 T cells fail to develop into follicular T helper (TFH) cells and to help B cells mature into germinal center B cells after viral infection. We found that mice lacking STIM1/STIM2 in T cells cannot produce virus-specific antibodies upon infection with lymphocytic choriomeningitis virus (LCMV) or vaccination with influenza virus. Important goals of this application are to understand whether CRAC channels in TFH cells control pulmonary immune responses to infection with influenza and to characterize the molecular mechanisms by which CRAC channels control the development and function of TFH cells in influenza. Besides the CRAC channel, about 600 ion channels and transporters are expressed in mammals, but to date only a few are established to contribute to T cell-mediated immune responses. We hypothesize that other ion channels besides the CRAC channel play important roles in regulating TFH cell-dependent humoral immunity to influenza. However, no studies have systematically addressed this question so far, which is a major gap in our knowledge of T cell physiology and adaptive immunity. We will address this gap and systematically screen for and characterize ion channels that regulate TFH cell-dependent humoral immune responses to influenza in vivo. The long-term goal of our study is to identify ion channels and downstream molecules they regulate that can be targeted therapeutically to enhance humoral immunity to influenza infection and vaccination. Since many ion channels are plasma membrane proteins, they are accessible to small molecule inhibitors or biologicals such as monoclonal antibodies to modulate their function and immune responses to influenza.